Ion mobility spectrometers (IMS) are used to detect trace substances in the air. They are widely used especially for detecting explosives, illegal drugs, chemical warfare agents and toxic industrial gases. The characteristic assembly units of an ion mobility spectrometer are the ionization chamber, the drift chamber and detectors. The ionization chamber and the drift chamber are usually separated by a grid in conventional ion mobility spectrometers. The analyte molecules to be determined are converted into ions in the ionization chamber. The ions formed are transferred from the ionization chamber into the drift chamber as an ion swarm by the action of an electric field. The analyte ions fly through the drift chamber against the resistance of the drift gas under the effect of a high-voltage electric field and are detected by the detector, due to the differences in the mobilities of different ions, in a time-resolved manner, since different analyte ions show different interactions with the drift gas, and therefore they also have different flight times and can thus be separated from one another.
Ion mobility spectrometers in which the drift gas flows from the detector in the direction of the ionization chamber have been known. The analyte gas is ionized and flows in the direction of the grid within the ionization chamber. The ions formed are consequently moving with the analyte gas in the direction of the grid against the direction of flow of the drift gas and then to the detector under the effect of a high-voltage field (Spangler and Carrico, Int. J. Mass Spectrom. Ion Phys., 1983, 52, 627).
Unidirectional flow guiding is described by Eiceman in U.S. Pat. No. 4,777,363, in which the analyte gas is introduced into the device on the detector side and leaves the device on the ionization chamber side. The ionization takes place in the ionization chamber and ions are accelerated toward the detector against the analyte gas flow. The drift gas and the analyte gas are identical here.
Both systems require a uniform electric field within the drift chamber for the separation of the ions. This field is built up by a series of ring electrodes, which are all electrically insulated. The necessary high voltage is usually 2,000-3,000 V. Such systems are very expensive, complicated to manufacture and can only be miniaturized with difficulty.
Contrary to the above-described IMS, it is, furthermore, known that the ions to be separated can be guided unidirectionally with the drift gas flow. The ions can be deflected from this direction of flow by a relatively low voltage. If they then reach electrodes, which are formed by the walls, they can be discharged, and a current can be measured. The drift gas and the analyte gas are identical here.
Such a system is found in so-called electron capture detectors. An early example is disclosed by Lovelock in U.S. Pat. No. 3,870,888. Total ionic currents can be measured with such systems. By contrast, it is not possible to distinguish different ion species.
It is known that long-lived ions can be separated from short-lived ones by extending the drift paths, e.g., by installing baffles. This principle is described, for example in connection with the detection of chemical warfare agents (U.S. Pat. Nos. 3,835,328, 4,075,550 as well as 5,223,712). The separation efficiency of such systems is relatively low, which may lead relatively frequently to the triggering of a false alarm.
An improvement is described in U.S. Pat. No. 5,047,723 by Puumalainen. The gas flow to be analyzed is first ionized here and then passed through a series of electric deflecting fields. Depending on the type of the ions, the ions are discharged at different electrodes. The current is measured and is an indicator of analytes that are present.
In WO 9416320, Paakanen et al. modified such a system once again and identified substances on the basis of their characteristic patterns, which are obtained from a plurality of electrodes connected in series at closely spaced locations from one another due to ion discharge. Besides ion signals, signals of semiconductor sensors are also included in a pattern recognition.
Furthermore, it is known that the last-named system can be improved by heating the analyte gas before the analysis and by the sensor electrodes forming multidimensional arrays (US 2003/0155503 A1). The signal evaluation is based on pattern recognition in this case as well. The drawback that the measuring system must first learn the particular pattern, i.e., that an extremely great calibrating effort is necessary, is associated with this. This applies especially to mixtures. Mixtures that are not taken into account, i.e., for example, combinations of analytes to be monitored with unknown impurities, may lead to false alarms or hinder the detection of the analytes to be monitored.
Finally, it is known that the analyte ions can be deflected by a high-frequency alternating field, to which a low compensation voltage is superimposed. The analyte ions are likewise transported here in a system in the direction of the drift gas (U.S. Pat. No. 6,495,823). A defined analyte ion species is let through the system and reaches the detector under defined conditions of the alternating field and the compensation voltage only. These ion sensors, which can be manufactured in a compact form, can be combined in arrays. However, such systems are expensive and extremely susceptible to ambient effects, such as pressure and moisture.